The composition of exhaust produced by the combustion of hydrocarbon fuels is a complex mixture of oxide gases (NOx, SOx, CO2, CO, H2O), unburned hydrocarbons, and oxygen. Measurement of the concentration of these individual exhaust gas constituents in real time can be used to improve combustion efficiency and lower emissions of polluting gases. Various devices have been proposed to operate as exhaust gas sensors that have the capability of measuring the concentration of a constituent within an exhaust stream.
One gas sensor known in the art is configured as a flat plate multilayer ceramic package designed to include a sensing component and a heating component. The sensing component includes one or more electrodes (e.g., a reference electrode and a sensing electrode) disposed on opposing sides of an electrolyte substrate. The heating component includes a heating element bonded to an electrically insulating substrate. The sensing component and the heating component are shaped from green sheets of the electrolyte and insulating materials (typically zirconia and alumina), respectively, which are stacked together and then sintered to bond the two components.
During operation of the sensor, the heating component is energized to raise the temperature of the sensing component to within an optimum operational range. When the temperature of the sensing component is within this range, the sensing component may be sufficiently accurate at detecting the presence of a particular exhaust constituent. However, when outside of the temperature range, signals generated by the sensing component may be unreliable. Unfortunately, it can be difficult to accurately control operation of the heater such that the temperature of the sensing component remains within the optimum operational range.
One way to improve heater control is disclosed in US Patent Application Publication No. 2007/0125647 (the '647 publication) by Wang et al. published Jun. 7, 2007. Specifically, the '647 publication discloses a sensor having a sensing element configured to detect an exhaust gas species such as NOx, and a heater co-fired with the sensing element. An RTD is glass bonded to the heater side of the already sintered sensor for use in sensing and controlling a temperature of the heater. Based on a current or voltage signal from the RTD indicative of a temperature of the heater, a current applied to the heater is varied so as to maintain the sensing element at a desired temperature.
Although perhaps an improvement over previous sensors that did not include an RTD, the gas sensor of the '647 publication may be expensive and still have reduced reliability and applicability. That is, because the RTD is glass bonded to an already sintered sensor, an extra assembly step is required that may increase assembly cost and assembly time. In addition, the glass bond may be prone to cracking when exposed to temperature gradients or extreme temperatures. And, as the RTD can only be placed outward of the heater when bonded separately, readings of the RTD may not accurately reflect the actual temperatures experienced by the sensing element.
The disclosed gas sensors are directed at solving one or more of the problems set forth above.